In chip on film (COF) package technology, a chip is flipped and bonded to a thin film, and each of bumps on the chip is connected to each of leads on the thin film. The chip is electrically connected to the thin film via the bumps for direct signal transmission, so can reduce the size of the whole package significantly. Owing to the thin film is made of flexible material, the thin film is also called as flexible substrate or flexible circuit board. With the advance of science and technology, conventional flexible circuit board has developed into fine pitch level (pitch is between about 20 μm and about 30 μm).
During the flexible circuit board developing toward super fine pitch level (pitch is between about 10 μm and about 20 μm), several problems need to be solved. Super fine pitch may cause the reduction of lead width due to etching process limitation, and the width reduction may generate insufficient bonding strength between the leads and the flexible substrate because of insufficient contact area. Furthermore, the flexible substrate heated in the bonding process may be expand to cause lead peeling from the flexible substrate or bonding shift between the corresponding bumps and leads.